Believe it or not, the quickest way is to walk if you don't mind a bit of a hike. There is a walkway that goes from Boardwalk Villas to MGM. Boardwalk Villas is itself a short walk from Epcot via the International Gate.
Buses generally do not run from park to park. (I've never heard of this happening but I don't know everything.) Taking two buses may take a long time. The boat is notoriously slow, and it makes stops at Boardwalk, Beach Club, and Dolphin before going to MGM.
I'd say leave the International Gate, and if you don't see a boat heading your way, start walking. Time depends on your walking style, but based on other people's comments I'd guess 20-30 minutes, maybe 15 for a serious hiker. The boat is likely to take 20-30 minutes even after it arrives.
Of course a car would be faster, but not enormously faster as you still have to get from Norway back your car, then into MGM from its parking lot.